Conventionally, optical glass having a high refractive index and with a high dispersion region are typically of a composition containing a large amount of lead oxide. Because of favorable glass stability and a low glass transition point (Tg), they have been used for precision mold press molding. For example, Patent Document 1 discloses an optical glass for a precision mold press which contains a large amount of lead oxide.
However, a reduction condition is maintained in order to prevent oxidation of the mold when molding by a precision mold press is performed. Therefore, when the glass component contains lead oxide, lead reduced from the glass surface breaks off and adheres to the mold surface. Accordingly, there have been problems in that the precision face of the mold is not maintained. Also, lead oxide is toxic to the environment so that glass free from lead oxide has been desired.
To meet such needs, many types of optical glass for press molding a having high refractive index with a high dispersion region, and not containing lead oxide have been developed. However, most of these types are phosphate glass that contain Nb2O5 at a high concentration. For example, Patent Document 2 and Patent Document 3 disclose P2O5—Nb2O5—WO3—(K2O, Na2O, Li2O) based glass, and Patent Document 4 discloses P2O5—Nb2O5TiO2—Bi2O3—Na2O based glass. However, many of these types of glass have a Tg that is said to be low, exceeding 480° C. Furthermore, these types of glass must include a large amount of Nb2O5 for achieving a high refractive index with high dispersion; therefore, they may be disadvantageous in that devitrification resistance is not that high.
In addition, compositions including a large amount of Bi2O3 have been known for glass having a low Tg. For example, Nonpatent Documents 1, 2, 3, 4, and 5 disclose Bi2O3—Ga2O3—PbO based glass, Bi2O3—Ga2O3—(Li2O, K2O, Cs2O) based glass, and Bi2O3—GeO2 based glass. Although these types of glass have Tg of 480° C. or lower, the absorption edge of the glass is longer than 450 nm. Hence, transparency in the visible region may be drastically reduced, and thus, these types of glass cannot be used as an optical lens required to have high transparency in the visible region.
[Patent Document 1] Japanese Patent Application Laid Open No. H1-308843
[Patent Document 2] Japanese Patent Application Laid Open No. 2003-321245
[Patent Document 3] Japanese Patent Application Laid Open No. H8-157231
[Patent Document 4] Japanese Patent Application Laid Open No. 2003-300751
[Nonpatent Document 1] Physics and Chemistry of Glasses, p 119, Vol. 27, No. 3, June 1986
[Nonpatent Document 2] American Ceramic Society, p 2315, Vol. 75, No. 9, October 1992
[Nonpatent Document 3] American Ceramic Society, p 1017, Vol. 77, No. 4, October 1994
[Nonpatent Document 4] American Ceramic Society Bulletin, p 1543, Vol. 71, No. 10, October 1992
[Nonpatent Document 5] Glass Technology, p 106, Vol. 28, No. 2, April 1987